1. Field of the Invention
The field of the invention comprises a soluble composition of matter and process for electrolytically depositing a chromium oxide coating on a metal substrate from a bath containing a trivalent chromium compound.
2. Description of the Related Art
The majority of tin mills produce Electrolytic Chromium Coated Steel (ECCS) from chromium plating baths based on hexavalent chromium. Although the chromium layer provides protection for the steel or tin layer or zinc layer on the steel, the surface of the chromium is not especially suitable for applying other coatings since it is difficult to get other materials to adhere to it. Accordingly, the chromium metal is converted into a chromium oxide to promote adhesion. Strong oxidizing solutions such as hexavalent chromium solutions make a thin oxide on chromium automatically. A second step may be used for better control.
One of the difficulties with employing hexavalent chromium compounds in electrolytic coating baths for this process is that it is considered carcinogenic, teratogenic and toxic. As a result, use of these baths present occupational and environmental problems. Employing safe equipment such as ventilating and recovery systems to prevent atmospheric and water pollution as well as safe operating procedures that require highly trained and skilled operators minimize or avoid these problems.
Trivalent chromium compounds substantially eliminate or minimize occupational and environmental problems associated with hexavalent chromium. Trivalent chromium solutions, however, do not form oxide while plating the metal using prior art processes.
The prior art in one instance, teaches that the electrolytic deposition of chrome oxides from trivalent chromium baths proceeds in two steps, the first of which involves electrolytic deposition of chromium metal from a trivalent chromium bath, the second, a conversion of the chromium metal coating to a chromium oxide compound.
Specifically in this regard, Lavezzari, U.S. Pat. No. 4,520,077 describes both a two-step and a so-called "one-step" process for depositing chromium metal and chromium oxide from a trivalent chromium bath. The reaction deposits chromium metal and afterwards trivalent chromium in the bath also reacts to form a chromium hydroxide on the deposited chromium metal. A subsequent dehydration or oxidation process converts the chromium hydroxide to a chromium oxide. The patentee specifies that the electrolytically deposited trivalent chromium film transforms chemically into chromium hydroxide by an optimal combination of at least the electrolytic bath composition, temperature, types of anodes, and cathodic current density.
The one-step process of Lavezzari is directed to the formation of a two layer coating of chromium metal with a chromium hydroxide top coat in a single bath. The two-stage process deposits a chromium metal first coat in one bath and a chromium hydroxide coating in a second bath utilizing the same chemistry. In both the one-stage and two-stage processes, a boron oxide such as boric acid is added to the coating bath.
Shahin, U.S. Pat. No. 5,294,326 describes a composition for applying chromium metal from a trivalent chromium electrolytic coating bath which requires boric acid anywhere from about 50 grams per liter of the bath up to its solubility limit in the bath.
McMullen et al., U.S. Pat. No. 4,450,052 also describe conventional trivalent chromium plating baths for the deposition of chromium metal which also contain boric acid. Lashmore et al., U.S. Pat. No. 4,804,446; Lashmore, U.S. Pat. No. 4,461,680 and Huba et al., U.S. Pat. No. 3,706,641 all describe electrodeposition of chromium metal from a trivalent chromium metal electrolyte which also employ boric acid as a component in the bath.
Benaben et al., U.S. Pat. No. 4,612,091 describe a chromium electroplating bath based on trivalent chromium which is not chelated whereas, Tardy et al., U.S. Pat. No. 4,460,438 describe a composition and a process for the electrolytic deposit of chromium from a trivalent chromium bath obtained by the reduction of chromic acid in a sulfuric medium by means of an excess of a reducing alcohol such as methanol.
The high speed electrolytic coating of steel, or other metals used on an industrial scale, requires high current densities. Industry presently uses current densities somewhere in the range of about 800 amps per square foot (ASF) and seeks the advantage of a composition and a process for forming chromium oxide coatings on steel or other metals at this or higher current densities. Higher current densities would increase production rates or line speeds if bath compositions were available that would allow plating at these conditions.
Industry also seeks the advantage of directly obtaining chromium oxide coatings from trivalent chromium compositions which have high surface area and may chemically bond to the coating so that other coatings such as organic coatings e.g., epoxy coatings, phenolic coatings and buff-vinyl coatings would adequately adhere to the chromium oxide substrate.
Manufacturers also want to obtain the advantage of a composition and a method for electrolytically depositing chromium coatings from trivalent chromium compositions at plating efficiencies of from about 30 to about 40 percent or greater, and at current densities from about 500 to about 1000 ASF.
The coating industry also wants the advantage of a composition and a process for depositing chromium oxide coatings in an amount up to or greater than about 2 mg/ft.sup.2 and especially coating weights greater than about 0.4 mg/ft.sup.2 that will provide excellent adhesion of coatings such as organic coatings e.g., epoxy coatings, phenolic coatings and buff-vinyl coatings and other coatings known in the art.
Jones and Shahin in application Ser. No. 08/469,020 filed Jun. 6, 1995, now abandoned, describe a process for obtaining chromium oxide coatings from a trivalent chromium compound. It was discovered in this process that boric acid and similar boron oxide compounds act as buffering agents to stabilize the pH of the chrome plating bath composition during the electroplating operation. Stabilizing the pH of the bath at somewhere around 2.5 promotes the deposition of chrome metal in the electrolytic coating process. The buffering agents substantially minimize or eliminate any increases in pH that occur in the cathode film of the cell. Jones and Shahin, however, eliminated added buffers or boron oxide compounds from the trivalent chromium oxide composition in order to make the pH increase faster in the cathode film. This faster increase in pH allows for the direct formation of chrome oxides on the cathode. It was found that the trivalent chromium at higher pH's formed oligomers unlike hexavalent chromium compounds, and readily plated on most metallic surfaces directly to form a chromium oxide during the plating process.
Electrolytic chromium coated steel (ECCS) sometimes referred to as tin free steel or TFS, as described by Shahin in U.S. Pat. No. 5,294,326 applied from trivalent chromium baths avoid the problems associated with hexavalent chromium. The trivalent chromium baths, however, contain boric acid or boron oxide compounds or other similar buffering agents.
Employing a chromium metal-chromium oxide two bath high speed production line, in which the first bath contains TFS trivalent chromium and a boron oxide buffering agent as described by Shahin, and the second trivalent chromium free of boron oxide buffers to promote chromium oxide production, as described by Shahin and Jones, encounters a problem because of the high line speeds employed in the TFS manufacturing process. There is considerable drag-out from the vessel containing the first plating bath into the vessel containing second plating bath used to deposit an oxide film. Running the production line for some time, drags or introduces boric acid or other boron oxide compounds or buffering agents into the second plating vessel which must be free of these compounds in order to deposit a chrome oxide film. As a result chromium deposition on the substrate increases and protective chromium oxide film production decreases or is substantially terminated.
It would be an advantage, therefore, to provide a process and a composition that eliminates or minimizes the buildup of boric acid, other boron oxide compounds or other buffering agents in the second plating vessel as a result of this drag-out.
These and other advantages are obtained according to the present invention which comprises a composition, process and product obtained by the process which substantially obviates one or more of the limitations and disadvantages of the described prior compositions, processes and products.
The present invention comprises a water soluble composition and a process for electrolytically depositing chromium oxide coatings directly from trivalent chromium as well as a product produced by the process in which the foregoing and other disadvantages are overcome.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the composition, process and product obtained by the process, particularly pointed out in the written description and claims hereof.